A typical antenna for wireless devices (such as for instance, and without limitation, a handset, a mobile phone, a smartphone, a PDA, a MP3 player, a headset, a USB dongle, a laptop, a PCMCIA or a Cardbus 32 card), comprises a conductive plate or wire usually mounted on a carrier made of plastic (such as for instance Poly Carbonate, Liquid Crystal Polymer, Poly Oxide Methylene, PC-ABS, or PVC) that provides mechanical support.
The antenna is assembled in the wireless device, forming an integral part of the device. The wireless device will usually comprise a multilayer printed circuit board (PCB) on which it carries the electronics. One of the layers of the multilayer PCB typically serves as a ground plane of the antenna.
One way of contacting the antenna is by means of a spring contact. A spring contact comprises a strip or similar of a conductive material (typically, metal) that includes one or several bends forming a spring (i.e., a structure capable of exerting a tensional strength when pressure is applied to it). When the antenna is assembled onto the PCB of the wireless device, the mechanical interference of the tip of the spring contact with the PCB results in the spring contact applying a tensional strength on the landing area of the PCB (such as, for example, a pad), ensuring good electrical continuity between the antenna and the relevant tracks in the PCB.
In some cases the spring contact is used to feed the antenna, establishing an electrical path to connect the antenna with a radio frequency (RF) front-end of the circuit, or an RF input/output of an electronic device, on the PCB. In other cases, the spring contact is used to connect the antenna to the ground plane of the PCB, which can be advantageous to tailor the input impedance of the antenna, or the resonant modes of the antenna, or a combination of both effects.
Usually, the landing area of a spring contact on the PCB of the wireless device is substantially close to an edge of the PCB (for example, the top edge of the PCB in a handset). Such an arrangement is preferable because a resonant mode of the antenna can advantageously excite currents on the ground plane of the PCB that flow along the entire length of said ground plane, enhancing the radiation process. This is particularly interesting for small-sized handsets (such as, for instance, bar-type, clamshell-type, slider-type or swivel-type handsets), because of the reduced dimensions of the ground plane. The requirement of feeding the antenna close to an edge of the PCB makes it advantageous to provide the spring contacts of the antenna at points close to the perimeter of the conductive plate of the antenna.
A typical process used for the fabrication of antennas for wireless devices comprises the steps of stamping a flat solid plate of conductive material (such as, for example, copper, aluminum, brass, silver, gold, or some other type of good conducting alloy) to cut the shape of the perimeter of the antenna out of the original flat solid plate. The resulting piece of conductive material is a flat structure. Pressure can then be applied to the structure in one or several steps, to bend portions of the piece of conductive material and define the three-dimensional structure of the antenna (such as for example to create capacitive loading elements, or to conform the conductive plate to a plastic carrier, or to a plastic cover, or chassis, of a wireless device).
When an antenna comprises one or more spring contacts, the stamping process defines a shape of the perimeter of the antenna including strips protruding from the main body of the antenna. The strips will then be bent in order to provide the adequate shape to the spring contacts.
In general, when fabricating an antenna comprising one or several spring contacts by means of a process involving the step of stamping of a plate of conductive material, the area of the smallest possible rectangle that completely encloses the perimeter of the main body of the antenna and the strips of the spring contacts (hereinafter also referred to as the antenna total area) will be significantly larger than the area of the smallest possible rectangle that completely encloses the perimeter of the main body of the antenna but not necessarily the strips of the spring contacts (hereinafter also referred to as the antenna body area). In the context of this patent application, the stamping area overhead is defined as the difference between the antenna total area and the antenna body area.
For illustration purposes, and without any limitation, FIG. 1 presents an example of an antenna fabricated by stamping a plate of a conductive material. The antenna comprises a main body (100) and two strips, labeled as (101) and (102), that will be used to create two spring contacts. FIG. 1a depicts the antenna as a flat structure, before bending the strips (101, 102) to form the spring contacts (see FIG. 1b). In FIG. 1a, the main body (100) and the strips (101, 102) are coplanar. The smallest possible rectangle that encompasses the perimeter of the antenna, including both the perimeter of the main body (100) and that of the strips (101, 102), is indicated with reference numeral (104). The smallest possible rectangle that encompasses the perimeter of the main body of the antenna (100), not necessarily including the strips (101, 102), is indicated with reference numeral (103). From the figure, it is clear that the area of rectangle (103) (i.e., the antenna body area) is smaller than the area of rectangle (104) (i.e., the antenna total area), this difference being the stamping area overhead. The stamping area overhead of the antenna is due to the fact that the strips (101, 102) protrude from the perimeter of the main body of the antenna (100) towards the outside, and this overhead implies an additional rectangular area of conducting plate for the stamping process of the antenna, which in turn translates into extra costs. Moreover, this additional area of conducting plate is used very inefficiently, as only the portion corresponding to the strips (101) and (102) will be retained after the stamping process, while the rest of the material will be discarded.
Some attempts have been made to try to reduce the stamping area overhead of the antenna (and hence the cost associated to using an additional amount of conductive material) by designing the spring contacts in such a way that the antenna total area is approximately the same as the antenna body area.
In these cases, such as for instance the example illustrated in FIG. 2, the geometry of the main body of the antenna (200) is modified in the region (203), in which the strips of spring contacts (201, 202) are connected to the main body (200). The shape of the main body of the antenna (200) recedes in that region (203) to allow the conducting strips of the spring contacts (201, 202) to be placed without extending beyond the minimum rectangle (205) that encompasses the perimeter of the main body of the antenna (200).
However, when folding the strips (201, 202) to shape the spring contacts (as depicted in FIG. 2b), the projection of the strips (201, 202) on the PCB on which the antenna is mounted will be shorter than the original length of the unfolded strips (201, 202), which means that the landing area of the spring contacts on said PCB will not occur near the edge of the PCB (assuming that the main body of the antenna does not extend beyond said edge). In the context of this document, by the term “projection” it is understood the orthogonal projection on the plane defined by a PCB of the handset or wireless device.
To keep the landing area of the spring contacts near the edge of the PCB, the antenna must be displaced parallel to the plane of the PCB until the landing area of the spring contacts is substantially close to the edge of the PCB, but this means that a portion of the antenna has a projection beyond the edge of the PCB, thus making the device larger unless said portion of the antenna is folded downwards forming a capacitive load. For example, such a portion (204) of the antenna in FIG. 2a has been bent approximately 90 degrees in FIG. 2b to allow the spring contacts (201, 202) to land near an edge of a PCB, without said portion (204) extending beyond said edge. However, this solution presents some important limitations. For example, the mechanical design of the spring contact cannot be treated independently from the electrical design of the antenna. A change in the height of the antenna to increase the bandwidth, or in the length of the capacitive element (204) to tune the operating bands, will make it necessary to redesign the spring contact, and modify the length of the strips (201) and (202). Similarly, a change in the shape of the spring contact to increase the tensional strength exerted on the landing area of the PCB, will make it necessary to modify the electrical design of the antenna, for instance the length of the capacitive loading element (204), in order not to increase the antenna total area with respect to the antenna body area, and incur in a stamping area overhead.
In the examples of antennas with spring contacts shown in FIGS. 1 and 2, the strips of conductive material that will be used to create the spring contacts (101, 102, 201, 202) protrude from the main body of the antenna (100, 200) towards the outside, which is clearly different from the antennas with inner spring contacts of the present invention.
The present invention discloses a novel type of antennas that comprise an inner spring contact. According to the present invention the inner spring contact allows to feed the antenna at an edge of the PCB on which the antenna is mounted, while avoiding substantially any stamping area overhead.
Space Filling Curves
In some examples, the antenna may be miniaturized by shaping at least a portion of the conducting trace, conducting wire or contour of a conducting sheet of the antenna (e.g., a part of the arms of a dipole, the perimeter of the patch of a patch antenna, the slot in a slot antenna, the loop perimeter in a loop antenna, or other portions of the antenna) as a space-filling curve (SFC).
A SFC is a curve that is large in terms of physical length but small in terms of the area in which the curve can be included. More precisely, for the purposes of this patent document, a SFC is defined as follows: a curve having at least five segments, or identifiable sections, that are connected in such a way that each segment forms an angle with any adjacent segments, such that no pair of adjacent segments defines a larger straight segment. In addition, a SFC does not intersect with itself at any point except possibly the initial and final point (that is, the whole curve can be arranged as a closed curve or loop, but none of the lesser parts of the curve form a closed curve or loop). A SFC can comprise straight segments, curved segments, or a combination of both.
A space-filling curve can be fitted over a flat or curved surface, and due to the angles between segments, the physical length of the curve is larger than that of any straight line that can be fitted in the same area (surface) as the space-filling curve. Additionally, to shape the structure of a miniature antenna, the segments of the SFCs should be shorter than at least one fifth of the free-space operating wavelength, and possibly shorter than one tenth of the free-space operating wavelength. The space-filling curve should include at least five segments in order to provide some antenna size reduction, however a larger number of segments may be used. In general, the larger the number of segments and the narrower the angles between them, the smaller the size of the final antenna.
Box-Counting Curves
In other examples, the antenna may be miniaturized by shaping at least a portion of the conducting trace, conducting wire or contour of a conducting sheet of the antenna to have a selected box-counting dimension.
For a given geometry lying on a surface, the box-counting dimension is computed as follows. First, a grid with substantially squared identical cells boxes of size L1 is placed over the geometry, such that the grid completely covers the geometry, that is, no part of the curve is out of the grid. The number of boxes N1 that include at least a point of the geometry are then counted. Second, a grid with boxes of size L2 (L2 being smaller than L1) is also placed over the geometry, such that the grid completely covers the geometry, and the number of boxes N2 that include at least a point of the geometry are counted. The box-counting dimension D is then computed as:
  D  =      -                            log          ⁡                      (                          N              ⁢                                                          ⁢              2                        )                          -                  log          ⁡                      (                          N              ⁢                                                          ⁢              1                        )                                                log          ⁡                      (                          L              ⁢                                                          ⁢              2                        )                          -                  log          ⁡                      (                          L              ⁢                                                          ⁢              1                        )                              
For the purposes of the antenna with at least one inner spring contact described herein, the box-counting dimension may be computed by placing the first and second grids inside a minimum rectangular area enclosing the conducting trace, conducting wire or contour of a conducting sheet of the antenna and applying the above algorithm. The first grid should be chosen such that the rectangular area is meshed in an array of at least 5×5 boxes or cells, and the second grid should be chosen such that L2=½L and such that the second grid includes at least 10×10 boxes. The minimum rectangular area is an area in which there is not an entire row or column on the perimeter of the grid that does not contain any piece of the curve. Further, the minimum rectangular area preferably refers to the smallest possible rectangular area that completely encloses the curve.
The desired box-counting dimension for the curve may be selected to achieve a desired amount of miniaturization. The box-counting dimension should be larger than 1.1 in order to achieve some antenna size reduction. If a larger degree of miniaturization is desired, then a larger box-counting dimension may be selected, such as a box-counting dimension ranging from 1.5 to 3. For the purposes of this patent document, curves in which at least a portion of the geometry of the curve, or the entire curve, has a box-counting dimension larger than 1.1 are referred to as box-counting curves.
For very small antennas, for example antennas that fit within a rectangle having maximum size equal to one-twentieth the longest free-space operating wavelength of the antenna, the box-counting dimension may be computed using a finer grid. In such a case, the first grid may include a mesh of 10×10 equal cells, and the second grid may include a mesh of 20×20 equal cells. The box-counting dimension (D) may then be calculated using the above equation.
In general, for a given resonant frequency of the antenna, the larger the box-counting dimension, the higher the degree of miniaturization that will be achieved by the antenna. One way to enhance the miniaturization capabilities of the antenna is to arrange the several segments of the curve of the antenna pattern in such a way that the curve intersects at least one point of at least 14 boxes of the first grid with 5×5 boxes or cells enclosing the curve. If a higher degree of miniaturization is desired, then the curve may be arranged to cross at least one of the boxes twice within the 5×5 grid, that is, the curve may include two non-adjacent portions inside at least one of the cells or boxes of the grid.
FIG. 9 illustrates an example of how the box-counting dimension of a curve (900) is calculated. The example curve (900) is placed under a 5×5 grid (901) (FIG. 9 upper part) and under a 10×10 grid (902) (FIG. 9 lower part). As illustrated, the curve (900) touches N1=25 boxes in the 5×5 grid (901) and touches N2=78 boxes in the 10×10 grid (902). In this case, the size of the boxes in the 5×5 grid (901) is twice the size of the boxes in the 10×10 grid (902). By applying the above equation, the box-counting dimension of the example curve (900) may be calculated as D=1.6415. In addition, further miniaturization is achieved in this example because the curve (900) crosses more than 14 of the 25 boxes in grid (901), and also crosses at least one box twice, that is, at least one box contains two non-adjacent segments of the curve. More specifically, the curve (900) in the illustrated example crosses twice in 13 boxes out of the 25 boxes.
Grid Dimension Curves
In further examples, the antenna may be miniaturized by shaping at least a portion of the conducting trace, conducting wire or contour of a conducting sheet of the antenna to include a grid dimension curve.
For a given geometry lying on a planar or curved surface, the grid dimension of curve may be calculated as follows. First, a grid with substantially identical cells of size L1 is placed over the geometry of the curve, such that the grid completely covers the geometry, and the number of cells N1 that include at least a point of the geometry are counted. Second, a grid with cells of size L2 (L2 being smaller than L1) is also placed over the geometry, such that the grid completely covers the geometry, and the number of cells N2 that include at least a point of the geometry are counted again. The grid dimension D is then computed as:
  D  =      -                            log          ⁡                      (                          N              ⁢                                                          ⁢              2                        )                          -                  log          ⁡                      (                          N              ⁢                                                          ⁢              1                        )                                                log          ⁡                      (                          L              ⁢                                                          ⁢              2                        )                          -                  log          ⁡                      (                          L              ⁢                                                          ⁢              1                        )                              
For the purposes of the antenna with at least one inner spring contact described herein, the grid dimension may be calculated by placing the first and second grids inside the minimum rectangular area enclosing the curve of the antenna and applying the above algorithm. The minimum rectangular area is an area in which there is not an entire row or column on the perimeter of the grid that does not contain any piece of the curve. Further the minimum rectangular area preferably refers to the smallest possible rectangular area that completely encloses the curve.
The first grid may, for example, be chosen such that the rectangular area is meshed in an array of at least 25 substantially equal cells. The second grid may, for example, be chosen such that each cell of the first grid is divided in 4 equal cells, such that the size of the new cells is L2=½ L1, and the second grid includes at least 100 cells.
The desired grid dimension for the curve may be selected to achieve a desired amount of miniaturization. The grid dimension should be larger than 1 in order to achieve some antenna size reduction. If a larger degree of miniaturization is desired, then a larger grid dimension may be selected, such as a grid dimension ranging from 1.5-3 (e.g., in case of volumetric structures). In some examples, a curve having a grid dimension of about 2 may be desired. For the purposes of this patent document, a curve or a curve where at least a portion of that curve is having a grid dimension larger than 1 is referred to as a grid dimension curve.
In general, for a given resonant frequency of the antenna, the larger the grid dimension the higher the degree of miniaturization that will be achieved by the antenna. One example way of enhancing the miniaturization capabilities of the antenna is to arrange the several segments of the curve of the antenna pattern in such a way that the curve intersects at least one point of at least 50% of the cells of the first grid with at least 25 cells enclosing the curve. In another example, a high degree of miniaturization may be achieved by arranging the antenna such that the curve crosses at least one of the cells twice within the 25-cell grid, that is, the curve includes two non-adjacent portions inside at least one of the cells or cells of the grid.
An example of a grid-dimension curve is given in FIG. 10. In FIG. 11 it is shown how this curve of FIG. 10 is placed in a 4×8 grid with 32 cells. The curve crosses all 32 cells and therefore N1=32. In FIG. 12 the curve of FIG. 10 is shown in combination with an 8×16 grid with 128 cells. The curve crosses all 128 cells and therefore N2=128. The resulting grid-dimension is therefore 2. In FIG. 13 the curve of FIG. 10 is shown placed in a 16×32 grid with 512 cells. The curve crosses at least one point of 509 cells.
Multilevel Structures
In some examples, at least a portion of the conducting trace, conducting wire or conducting sheet of the antenna may be coupled, either through direct contact or electromagnetic coupling, to a conducting surface, such as a conducting polygonal or multilevel surface. Further the curve of the antenna may include the shape of a multilevel structure. A multilevel structure is formed by gathering several geometrical elements, such as polygons or polyhedrons, of the same type or of different type (e.g., triangles, parallelepipeds, pentagons, hexagons, circles or ellipses as special limiting cases of a polygon with a large number of sides, as well as tetrahedral, hexahedra, prisms, dodecahedra, etc.) and coupling electromagnetically at least some of such geometrical elements to one or more other elements, whether by proximity or by direct contact between elements.
At least two of the elements may have a different size. However, also all elements may have the same or approximately the same size. The size of elements of different a type may be compared by comparing their largest diameter.
The majority of the component elements of a multilevel structure have more than 50% of their perimeter (for polygon and surface like elements) or their surface (for polyhedrons) not in contact with any of the other elements of the structure. Thus, the component elements of a multilevel structure may typically be identified and distinguished, presenting at least two levels of detail: that of the overall structure and that of the polygon or polyhedron elements that form it. Additionally, several multilevel structures may be grouped and coupled electromagnetically to each other to form higher-level structures. In a single multilevel structure, all of the component elements are polygons with the same number of sides or are polyhedrons with the same number of faces. However, this characteristic is not present when several multilevel structures of different natures are grouped and electromagnetically coupled to form meta-structures of a higher level.
A multilevel antenna includes at least two levels of detail in the body of the antenna: that of the overall structure and that of the majority of the elements (polygons or polyhedrons) which make it up. This may be achieved by ensuring that the area of contact or intersection (if it exists) between the majority of the elements forming the antenna is only a fraction of the perimeter or surrounding area of said polygons or polyhedrons.
One example property of multilevel antennae is that the radioelectric behavior of the antenna can be similar in more than one frequency band. Antenna input parameters (e.g., impedance) and radiation pattern remain similar for several frequency bands (i.e., the antenna has the same level of adaptation or standing wave relationship in each different band), and often the antenna presents almost identical radiation diagrams at different frequencies. The number of frequency bands is proportional to the number of scales or sizes of the polygonal elements or similar sets in which they are grouped contained in the geometry of the main radiating element.
In addition to their multiband behavior, multilevel structure antennae may have a smaller than usual size as compared to other antennae of a simpler structure (such as those consisting of a single polygon or polyhedron). Additionally, the edge-rich and discontinuity-rich structure of a multilevel antenna may enhance the radiation process, relatively increasing the radiation resistance of the antenna and reducing the quality factor Q (i.e., increasing its bandwidth).
A multilevel antenna structure may be used in many antenna configurations, such as dipoles, monopoles, patch or microstrip antennae, coplanar antennae, reflector antennae, wound antennae, antenna arrays, or other antenna configurations. In addition, multilevel antenna structures may be formed using many manufacturing techniques, such as printing on a dielectric substrate by photolithography (printed circuit technique); dieing on metal plate, repulsion on dielectric, or others.